Vehicle accessories such as air conditioning systems, power steering systems and water pumps are typically driven directly by the engine crankshaft through, for example, a belt and pulley system. Therefore, the accessories must be capable of operating over the entire speed range of the engine, as the operating speeds of the accessories in a conventional drive system are directly proportional to the speed of the engine. The accessories are therefore typically designed to provide full capacity when the engine is operating at the low end of the engine speed range, because the accessories must be designed to give sufficient performance at low speeds, such as during engine idle, as well as being capable of running at high speeds during engine maximum speed operation. At higher engine speeds, excess energy is transferred to the accessories that may be lost.